Short answer
Plant outdoors after your last frost, once nights stay above about 10 °C (50 °F). At 35–50° latitude that usually means mid-May to mid-June in the Northern Hemisphere and mid-November to mid-December in the Southern. Grower practice. Home growing outdoors is legal only in some places: check local rules first.
Why
Cold sets plants back. Frost-tender seedlings need settled weather, and anything raised indoors needs hardening off before it goes out [2].
Day length decides what a photoperiod plant does. Cannabis measures the night; once nights pass a cultivar's threshold, shoot tips switch to flowers [3]. Some hemp cultivars had a critical day length of 13 h 45 min to 14 h [1].
Early spring days are short. At 45° latitude mid-April gives only about 13.4 h from sunrise to sunset (calculated), so a plant set out then may start flowering, then re-veg as days lengthen.
Autoflowers ignore day length. They flower by age, under a separate genetic pathway [4], so only temperature and season length limit them.
The numbers
| Latitude | Northern Hemisphere | Southern Hemisphere | Longest day (sunrise–sunset, calculated) |
|---|---|---|---|
| 25–30° | Late April–May (frost rarely limits) | Late October–November | ~13.7–14.1 h |
| 35–40° | Mid-May–early June | Mid-November–early December | ~14.5–15.0 h |
| 45–50° | Late May–mid-June | Late November–mid-December | ~15.6–16.4 h |
| 55° | Early–mid-June | Early–mid-December | ~17.4 h |
Your local last-frost date overrides this table.
What to do
- Find your average last-frost date and add a safety week.
- Wait for nights reliably above about 10 °C (50 °F).
- Start photoperiods indoors 3–6 weeks before, under long days.
- Harden off for 7–14 days.
- Plant out on a cloudy day or evening, and water in.
- For autos, sow straight into the final container or bed when nights are warm.
Common mistakes
- Planting photoperiods in April for a "head start", then losing weeks to re-veg.
- Skipping hardening off and bleaching the leaves.
- Planting late at high latitude, so flowering runs into autumn rain.
- Ignoring local rules on outdoor growing.
Related
- Full article: The cannabis life cycle, stage by stage
- Answers: Start indoors, move outside · When outdoor plants flower · Short northern seasons
- Terms: outdoor growing · hardening off · photoperiod
